Thermodynamics of solid and liquid aluminium

Based on thermodynamic calculations, it is shown that in the temperature range of 298–1273 K, heating and cooling of aluminum are thermodynamically equilibrium processes. When aluminum is heated, the molar volume energy of Gibbs decreases and the molar boundary energy of nanocrystals increases. When aluminum is cooled, the molar volume energy of Gibbs increases and the molar boundary energy of nanocrystals decreases. Liquid aluminum is a nanostructured system. Dendritic microcrystals are formed from nanocrystals. They play a large role in the processes of changing the structure of aluminum during its heating and cooling.

The intimate relationship between structural relaxation and the energy landscape of monatomic liquid metals

The characteristic property of a liquid, discriminating it from a solid, is its fluidity, which can be expressed by a velocity field. The reaction of the velocity field on forces is enshrined in the transport parameter viscosity. In contrast, a solid reacts to forces elastically through a displacement field, the particles are trapped in their potential minimum. The flow in a liquid needs enough thermal energy to overcome the changing potential barriers, which is supported through a continuous rearrangement of surrounding particles. Cooling a liquid will decrease the fluidity of a particle and the mobility of the neighbouring particles, resulting in an increase of the viscosity until the system comes to an arrest. This process with a concomitant slowing down of collective particle rearrangements might already start deep inside the liquid state. The idea of the potential energy landscape provides an attractive picture for these dramatic changes. However, despite the appealing idea there is a scarcity of quantitative assessments, in particular, when it comes to experimental studies. Here we present results on a monatomic liquid metal through a combination of ab initio molecular dynamics, neutron spectroscopy and inelastic x-ray scattering. We investigated the collective dynamics of liquid aluminium to reveal the changes in dynamics when the high temperature liquid is cooled towards solidification. The results demonstrate the main signatures of the energy landscape picture, a reduction in the internal atomic structural energy, a transition to a stretched relaxation process and a deviation from the high-temperature Arrhenius behavior of the relaxation time. All changes occur in the same temperature range at about $$1.4 \cdot T_{melting}$$ 1.4 · T melting , which can be regarded as the temperature when the liquid aluminium enters the landscape influenced phase and enters a more viscous liquid state towards solidification. The similarity in dynamics with other monatomic liquid metals suggests a universal dynamic crossover above the melting point.

On the question of using solid electrodes in the electrolysis of cryolite-alumina melts. Part 1.

This article is aimed at identifying issues associated with the use of solid cathodes in the electrolysis of cryolitealumina melts in order to determine conditions for their practical application. The contemporary technology of using solid anodes and cathodes is reviewed from its inception to the present time. The problems of stable electrolysis are discussed, such as effects of the electrode surface on the technological process. It is shown that all attempts undertaken over the recent 100 years to use solid electrodes, both reactive and inert, have been challenged with the emergence of electrolysis instability, formation of precipitates of varying intensity on the electrodes and impossibility of maintaining a prolonged process at current densities of above 0.4–0.5 A/cm2. Information is provided on the attempts to use purified electrolyte components with different ratios, metal-like and ceramic electrodes with a high purity and a smooth surface in order to approach real industrial conditions. However, to the best of our current knowledge, these experiments have not found commercial application. The authors believe that the most probable reason for the decreased current efficiency and passivation of solid electrodes consists in the chemical inhomogeneity and micro-defects of the bulk and surface structure of polycrystalline cathodes and anodes. It was the physical inhomogeneity of carbon electrodes that directed the development of the nascent electrolytic production of aluminium towards the use of electrolytic cells with a horizontal arrangement of electrodes and liquid aluminium as a cathode. This reason is assumed to limit the progress of electrolytic aluminium production based on the use of inert anodes and wettable cathodes in the designs of new generation electrolytic cells implying vertically arranged drained cathodes. The theoretical and experimental examination of this assumption will be presented in the following parts of the article.